1. Technical Field
The present invention relates generally to an apparatus and a method for using electron beams to microscopically inspect the surface of an object, and more particularly to inspect layers in a semiconductor device.
2. Discussion of the Prior Art
A variety of methods have been used to examine microscopic surface structures of semiconductors. These have important applications in the field of semiconductor chip fabrication, where microscopic defects at a surface layer make the difference between a good or bad chip. Holes or vias in an intermediate insulating layer often provide a physical conduit for an electrical connection between two outer conducting layers. If one of these holes or vias becomes clogged, it will be impossible to establish this electrical connection and the whole chip may fail. Examination of the microscopic defects in the surface of the semiconductor layers is necessary to ensure quality control of the chips.
Electron beams have several advantages over other mechanisms to examine samples. Light beams have an inherent resolution limit of about 100 nm-200 nm, but electron beams can investigate feature sizes as small as a few nanometers. Electron beams are manipulated fairly easily with electrostatic and electromagnetic elements, and are easier to produce and manipulate than x-rays.
Electron beams in semiconductor defect inspection do not produce as many false positives as optical beams. Optical beams are sensitive to problems of color noise and grain structures whereas electron beams are not. Oxide trenches and polysilicon lines are especially prone to false positives with optical beams due to grain structure.
A variety of approaches involving electron beams have been utilized for examining surface structure. In low-voltage scanning electron microscopy (SEM), a narrow beam of primary electrons is raster-scanned across the surface of a sample. Primary electrons in the scanning beam cause the sample surface to emit secondary electrons. Because the primary electrons in the beam of scanning electron microscopy are near a particular known electron energy (called xe2x80x98E2xe2x80x99), there is no corresponding charge build-up problem in SEM, and the surface of the sample remains neutral. However, raster scanning a surface with scanning electron microscopy is slow because each pixel on the surface is collected sequentially. Moreover, a complex and expensive electron beam steering system is needed to control the beam pattern.
Another approach is called Photo-Electron Emission Microscopy (PEM or PEEM), in which photons are directed at the surface of a sample to be studied, and by the photoelectric effect, electrons are emitted from the surface. On an insulating surface, the emission of these electrons, however, produces a net positive charge on the sample surface since there is a net flux of electrons from the surface. The sample continues to charge positively until there are no emitted electrons, or electrical breakdown occurs. This charge build-up problem limits the utility of PEEM for imaging insulators.
Another method of examining surfaces with electron beams is known as Low Energy Electron Microscopy (LEEM), in which a relatively wide beam of low-energy electrons is directed to be incident upon the surface of the sample, and electrons reflected from the sample are detected. However, LEEM suffers from a similar charge build-up problem since electrons are directed at the sample surface, but not all of the electrons are energetic enough to leave the surface. In LEEM, negatively-charged electrons accumulate on the surface, which repels further electrons from striking the sample, resulting in distortions and shadowing of the surface.
Several prior art publications have discussed a variety of approaches using electron beams in microscopy, but none have determined how to do so with parallel imaging at the same time the charge build-up problem is eliminated. One of these approaches is described by Lee H. Veneklasen in xe2x80x9cThe Continuing Development of Low-Energy Electron Microscopy for Characterizing Surfaces,xe2x80x9d Review of Scientific Instruments, 63(12), December 1992, pages 5513 to 5532. Veneklasen notes generally that the LEEM electron potential difference between the source and sample can be adjusted between zero and a few keV, but he does not recognize the charging problem or propose a solution to it. Habliston et al., in xe2x80x9cPhotoelectron Imaging of Cells: Photoconductivity Extends the Range of Applicability,xe2x80x9d Biophysical Journal, Volume 69, October 1995, pages 1615 to 1624, describe a method of reducing sample charging in photoelectron imaging with ultraviolet light.
Thus, there remains a need for a method utilizing electrons beams to investigate sample surfaces that eliminates the charge build-up problem and increases the speed of examining large sample surfaces.
The present invention provides an improved apparatus and method, called Secondary Electron Emission Microscopy (SEEM), for using electron beams to inspect samples with electron beam microscopy. The apparatus images a large number of pixels in parallel on a detector array, and thereby has the properties of being faster and lower in noise than conventional Scanning Electron Microscopes. Electron beam scanning systems are not required, and the electron beam current densities are not as high so that the probability of damaging sensitive samples is lessened.
The method of one embodiment of the invention comprises: providing a sample of a material having a characteristic energy value; directing an electron beam having a width appropriate for parallel multi-pixel imaging to be incident on the sample; and maintaining a stable electrostatic charge balance of the sample. (A xe2x80x98pixel,xe2x80x99 or picture element, is defined by the projected size of the image on an element of an electron detector.) One application of SEEM is the detection of defects in the manufacture of semiconductor devices. Another is for investigating other materials, including biological samples and tissues.
The electrons emitted from the sample are focused by a projection electron lens to an image plane and detected by an electron detector, which is preferably a time delay integrating (TDI) electron detector. The operation of an analogous TDI optical detector is disclosed in U.S. Pat. No. 4,877,326 to Chadwick et al, which is incorporated herein by reference. The image information may be processed directly from a xe2x80x98back thinxe2x80x99 TDI electron detector, or the emitted electrons may be converted into a light beam and detected with an optional optical system and a TDI optical detector.
The present invention overcomes many of the problems associated with prior art approaches to using electron beams for investigating sample surface structures by combining certain features of the LEEM and SEM techniques. Compared to the conventional Scanning Electron Microscope method of raster scanning an object, the invention utilizes a relatively wide beam of electrons to parallel-image the object. Essentially, a relatively wide beam of primary electrons is used as in LEEM, but the energies of these electrons are characteristic of those used in SEM. By operating the primary electron beam near energy E2 at a stable point on the yield curve of the sample material, the present invention realizes the unexpected advantage of eliminating the problem of charge build up on the sample surface associated with LEEM. The charge build-up on the surface of the object is controlled by directing the electron beam onto the object surface at an electron energy where the number of emitted secondary electrons equals the number of incident primary electrons.
SEEM is much faster than SEM because SEEM does not scan a narrow beam across the sample, but instead directs a relatively wide beam of electrons at the surface. To put this in numerical perspective, the spot size of the scanning beam in Scanning Electron Microscopy (SEM) is typically about 5 nanometers to 100 nanometers (5xc3x9710xe2x88x929 meters to 100xc3x9710xe2x88x929 meters). The spot size of the incident beam in Secondary Electron Emission Microscopy (SEEM) is about one millimeter to ten millimeters (1xc3x9710xe2x88x923 meters to 10xc3x9710xe2x88x923 meters). Thus, the spot size in SEEM is on the order of ten thousand to one million times larger than in SEM. Accordingly, SEEM is able to look at a larger surface more rapidly than is possible in SEM.
The primary electron energies in SEEM are close to the E2 point used in SEM, i.e. about 1-2 keV (one thousand electron volts). In LEEM, the primary electron energies are in the range of 0-100 eV below the E1 point for the material. Thus, the surface charges negatively.
The comparative speed advantage in SEEM, i.e. the maximum pixel rate, is limited mainly by the xe2x80x98dwell timexe2x80x99 and the xe2x80x98current density.xe2x80x99 The minimum dwell time that a beam must spend looking at a given image is determined by the acceptable Signal-to-Noise ratio of the image. The maximum current density is determined by such practical considerations as available gun brightness and possible sample damage. Because the focused beam of primary electrons in SEM must scan the beam across the entire surface to be inspected, the maximum practical pixel rate in Scanning Electron Microscopy is less than or equal to 100 million pixels/second (100 MHz). In Secondary Electron Emission Microscopy (SEEM), a large two-dimensional area of the sample is imaged in parallel without the need for scanning. The maximum pixel rate in SEEM is greater than 800 million pixels/second (800 MHz). The dwell time of the beam in SEEM may correspondingly be much longer than in SEM, and this permits a much lower current density while still maintaining a high Signal-to-Noise ratio. Thus, SEEM has the capability of investigating more sensitive sample surface structures while requiring lower brightness electron beam sources.